Separator for fuel cells

ABSTRACT

A separator for fuel cells is provided, which has excellent electrical conductivity, mechanical strength and gas impermeability. The separator is a molded article of a carbon-phenol resin molding compound obtained by reacting a phenol with an aldehyde in the presence of a catalyst, while mixing them with a carbon powder. As the catalyst, it is possible to use at least one selected from tertiary amines, carbonates, hydroxides and oxides of alkali metals or alkali earth metals. It is preferred that a content of nitrogen constituent in the molding compound is 0.3 wt % or less, and a carbon content in the molding compound is within a range of 75 wt % to 97 wt %.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a separator for fuel cells, andparticularly a separator for solid-polymer electrolyte type fuel cells,which is a molded article of a carbon-phenol resin molding compound.

[0003] 2. Disclosure of the Prior Art

[0004] In conventional fuel cells, a fuel gas containing hydrogen issupplied to an anode, and air containing oxygen is supplied to a cathodethat is spaced away from the anode by an electrolyte film, The followingelectrochemical reactions happen at the anode and cathode sides,respectively.

H₂→2H⁺+2e ⁻(½)O₂+2H⁺+2e ⁻→H₂O

[0005] Therefore, this fuel cell provides electric energy according tothe electrochemical reaction of H₂+(½)O₂H₂O.

[0006]FIG. 1 is an enlarged cross-sectional view of a solid-polymerelectrolyte type fuel cell. This fuel cell comprises an electrolyte film1 formed by an ion exchange membrane of a fluorinated resin, anode 2 andcathode 3 that are formed by a carbon cloth or a carbon paper, and apair of separators 4 having flow channels (5, 6). The anode and cathode(2, 3) are placed at both sides of the electrolyte film 1. Theseparators 4 are placed on the anode and cathode (2, 3) such that thefuel gas containing hydrogen is supplied to the anode 2 through the flowchannels 5 of one of the separators and the air containing oxygen issupplied to the cathode 3 through the flow channels 6 of the otherseparator. For the separators used in the conventional fuel cells,electrical conductivity and gas impermeability are importantcharacteristics. For example, as described in Japanese Patent EarlyPublications No. 11-195422, No. 2000-21421, and No. 2000-77079, it hasbeen proposed to use a molded article of a molding compound containing acarbon powder and a phenol resin as the separator.

[0007] However, in the conventional separators, there is a problem thatelectrical properties such as volume resistivity are not sufficientlysatisfied. That is, as a content of the carbon powder in the moldingcompound is increased to improve the electrical properties, a content ofthe phenol resin in the molding compound relatively decreases. In such acase, since a flowability of the molding compound In the molding stagelowers, there is a fear that clearances among carbon particles can notbe uniformly filled with the phenol resin, so that molded articleshaving residual pores are obtained. As a result, this leads to adecrease in mechanical strength and deterioration in gas impermeabilityof the separator.

SUMMARY OF THE INVENTION

[0008] Therefore, a primary concern of the present invention is toprovide a separator for fuel cells, which has excellent electricalconductivity, mechanical strength and gas impermeability.

[0009] That is, the separator of the present invention is a moldedarticle of a carbon-phenol resin molding compound obtained by reacting aphenol with an aldehyde in the presence of a catalyst, while mixing themwith a carbon powder.

[0010] It is preferred that the separator is of a thin-plate shapehaving a flow channel.

[0011] In addition, it is preferred that a carbon content in the moldingcompound is within a range of 75 wt % to 97 wt %.

[0012] As the catalyst used in the present invention, it is preferred touse at least one selected from tertiary amines, carbonates, hydroxidesand oxides of alkali metals or alkali earth metals.

[0013] Moreover, it is preferred that a content of nitrogen constituentin the molding compound is 0.3 wt % or less.

[0014] It is also preferred that the carbon powder contains 90 wt % ormore of fixed carbon.

[0015] These and still other objects and advantages of the presentinvention will become more apparent from the detail description andExamples of the present invention explained below.

[0016] The present disclosure relates to subject matter contained inJapanese Patent Application No. 2001-240254, filed on Aug. 8, 2001, thedisclosure of which is expressly incorporated herein by reference in itsentirety.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1 is an enlarged cross-sectional view of a solid-polymerelectrolyte type fuel cell.

DETAIL DESCRIPTION OF THE INVENTION

[0018] A separator for fuel cells of the present invention is a moldedarticle of a carbon-phenol resin molding compound obtained by reacting aphenol with an aldehyde in the presence of a catalyst, while mixing themwith a carbon powder. Since carbon particles work as nuclei during thereaction, a wet granular material of the carbon powder and the generatedphenol resin is obtained. By filtrating and drying the wet granularmaterial, it is possible to obtain a dry granular powder of thecarbon-phenol resin molding compound.

[0019] As the carbon powder used in the present invention, for example,it is possible to use natural graphite, artificial graphite, Kishgraphite, exfoliated graphite, carbon black, mesophase graphite, coke,charcoal, husk carbon, powder of carbon fiber or the like. In addition,it is preferred to use, the carbon powder containing 90 wt % or more offixed carbon. As the content of fixed carbon increases, the carbonpowder contains a higher amount of carbon, and impurities in the carbonpowder decrease. Therefore, the characteristics of the separator forfuel cells can be remarkably improved. An upper limit of the content offixed carbon is 100 wt %. A particle size of the carbon powder is notlimited. However, the carbon powder having a particle size of 1 to 200μm is preferably used.

[0020] In the present invention, as the phenol that is one of the basicingredients of the phenol resin, it is preferred to use a phenol havinghydrophobicity, which is hardly soluble in water. In addition, it ispreferred that a solubility in water of the hydrophobic phenol is 5 orless at normal temperature (30° C.). The term of “solubility in water”is defined as the maximum amount (g) of a solute that can be dissolvedin 100 g of water. Therefore, 5 or less of the solubility in water ofthe hydrophobic phenol means that a saturated state is achieved when 5 gof the hydrophobic phenol is dissolved in 100 g of water. When using thehydrophobic phenol, a lower limit of the solubility is zero.

[0021] Specifically, as the hydrophobic phenol, for example, it ispossible to use o-cresol, m-cresol, p-cresol, p-t-butyl phenol,4-t-butyl catechol, m-phenyl phenol, p-phenyl phenol, p-(α-cumyl)phenol, p-nonyl phenol, guaiacol, bisphenol-A, bisphenol-S, bisphenol-F,o-chloro phenol, p-chloro phenol, 2,4-dichloro phenol, o-phenyl phenol,3,5-xylenol, 2,3-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol,p-octylphenol, or the like. One of these compounds, or a combination oftwo or more of these compounds can be used as the hydrophobic phenol.

[0022] In addition, a phenol having hydrophilicity that a solubility inwater of the phenol is more than 5 at normal temperature (30° C.) mayused together with the hydrophobic phenol described above. As such awater-soluble phenol, for example, it is possible to use phenol,catechol, tannin, resorcin, hydroquinone, pyrogallol, or the like. Oneof these compounds, or a combination of two or more of these compoundscan be used as the hydrophilic phenol.

[0023] As the amount used of the hydrophobic phenol increases, an effectof preventing aggregation of the wet granular material of the carbonpowder and the phenol resin becomes higher. For example, it is preferredthat 5 wt % or more of the phenol used in the present invention is thehydrophobic phenol. When the amount used of the hydrophobic phenol isless than 5 wt %, the aggregation of the wet granular material mayoccurs. An upper limit of the amount used of the hydrophobic phenol is100 wt %.

[0024] As the aldehyde that is the other one of the basic ingredients ofthe phenol resin, it is particularly preferred to use formalin that isan aqueous solution state of formaldehyde. Alternatively, for example,it is possible to use trioxane, tetraoxane, paraformaldehyde, or thelike. In addition, at least a part of formaldehyde may be replaced withfurfural or furfuryl alcohol.

[0025] In the present invention, as the catalyst used for the additionalcondensation reaction between the phenol and the aldehyde, it ispossible to use a basic catalyst for synthesizing a resol-type phenolresin. However, it is needed to select the basic catalyst so as not toincrease a nitrogen content in the molding compound. That is, when acatalyst for synthesizing an ammonia resol-type phenol resin, i.e., anitrogen containing compound such as ammonia, primary amines orsecondary amines is used to synthesize the phenol resin, there is a fearthat a large amount of nitrogen impurities remains in the phenol resin.

[0026] As a content of the nitrogen impurities in the molding compoundincreases, the following disadvantage may occur. In general, it isneeded to run a cooling water having antifreeze property and lowelectrical conductivity in the fuel cell stack during the operation offuel cell. It is preferred that the electrical conductivity of thecooling water is maintained within the range of 200 μS/cm or less. Atthis time, the nitrogen impurities included in the separator areionized, so that the ionized nitrogen impurities elute into the coolingwater, This leads to an increase in electrical conductivity of thecooling water. As a result, the occurrence of electric leakage in thefuel cell and a reduction in EMF (electromotive force) of the fuel cellcome into problems. Thus, the reliability of the fuel cells may decreasebecause of the elution of nitrogen impurities from the separator.

[0027] From the above viewpoint, in the present invention, it ispreferred that a content of nitrogen constituent in the molding compoundis 0.3 wt % or less. Specifically, to reduce the content of nitrogenconstituent, it is preferred to use at least one selected fromcarbonates, hydroxides and oxides of alkali metal such as sodium,potassium, lithium and so on, carbonates, hydroxides and oxides ofalkali earth metal such as calcium, magnesium, barium and so on, andtertiary amines. For example, it is possible to use sodium hydroxide,potassium hydroxide, lithium hydroxide, sodium carbonate, calciumhydroxide, magnesium hydroxide, barium hydroxide, calcium carbonate,magnesium oxide, calcium oxide, trimethylamine, triethylamine,triethanolamine, 1,8-diazabicyclo [5,4,0] undecene-7, or the like.

[0028] These carbonates, hydroxides and oxides of the alkali metals orthe alkali earth metals contain no nitrogen constituent. On the otherhand, the tertiary amine contains the nitrogen constituent, but thenitrogen constituent is not incorporated into the phenol resin. Inaddition to the above-described components, a lubricant, fibers, epoxyresin, and/or a coupling agent may be blended, if necessary.

[0029] In the present invention, the phenol, aldehyde, catalyst, carbonpowder and the optional components are put in a reaction vessel. Inaddition, a sufficient amount of water for agitating is added into thereaction vessel. By agitating a resultant mixture in the reactionvessel, the additional condensation reaction between the phenol and thealdehyde proceeds.

[0030] At the initial stage of the reaction, the resultant mixture is ina mayonnaise-like state having poor viscosity. However, a flowability ofthe resultant mixture gradually increases by agitating. As the reactionfurther proceeds, a condensation product of the phenol and the aldehydeincluding the carbon powder begins to separate from water. Subsequently,a change of state suddenly happens. That is, a uniformly dispersed stateof agglomerated particles of the phenol resin generated by thecondensation reaction and the carbon powder is suddenly obtained in thereaction vessel.

[0031] After the reaction for generating the phenol resin reaches adesired level, the reaction product is cooled, and then agitating isstopped. Since the agglomerated particles settle down in water, it ispossible to readily separate the agglomerated particles from water byfiltration to obtain the wet granular material. In addition, waterremaining in the wet granular material can be readily removed by drying.As a result, a dry granular powder is obtained, which is a preferablestate in handling the molding compound of the present invention.

[0032] The obtained granular powder is characterized in that a ratio ofthe carbon powder and the phenol resin is substantially constant in eachparticle of the granular powder. Since the phenol resin that works as abinder provides an outermost layer with an extremely thin thickness ofthe granular powder, the carbon-phenol resin molding compound can beobtained with use of a reduced amount of the phenol resin. As a result,it is possible to relatively increase the content of the carbon powderin the molding compound.

[0033] Therefore, in the present invention, it is preferred that acontent of the carbon powder in the carbon-phenol resin molding compoundis 75 wt % or more. As the content of the carbon powder increases, theelectric conductivity of the separator obtained by molding thecarbon-phenol resin molding compound are further improved. On the otherhand, to maintain good mechanical strength and gas impermeability of theseparator, it is preferred that the content of the carbon powder in themolding compound is 97 wt % or less.

[0034] By the way, when using the hydrophobic phenol, the obtainedphenol resin becomes hydrophobicity as the additional condensationreaction proceeds. Therefore, the wet granular material that is amixture of the phenol resin and the carbon powder can be easily removedfrom water. In addition, since the reaction product has poorhygroscopicity and water absorbing property, it is possible to avoid theoccurrence of agglomeration at the time of filtrating the wet granularmaterial from water or drying the wet granular material to obtain thedry granular powder.

[0035] In the present invention, by molding the carbon-phenol resinmolding compound, a molded article of a thin-plate shape having requiredflow channels, through which the fuel gas including hydrogen or the airincluding oxygen passes, can be obtained as the separator. Therefore,fuel cells can be manufactured by use of the separator of the presentinvention.

[0036] As an example, a method of molding the carbon-phenol resinmolding compound to obtain the separator of the present invention isexplained. The molding compound is charged into a required die, and thenmolded at a heating temperature under pressure. For example, it ispreferred that the molding step is performed at a heating temperature of130 to 250° C. under a surface pressure of 10 to 200 MPa.

[0037] In particular, it is preferred that the separator is formed by atwo-stage molding method comprising the steps of: preparing acarbon-phenol resin molding compound by reacting a phenol with analdehyde in the presence of a catalyst, while mixing them with a carbonpowder such that a content of the carbon powder in the carbon-phenolresin molding compound is 75 wt % or more; pressing the molding compoundin a first die at a pre-molding temperature to obtain a pre-moldedarticle having a shape near the final shape of the separator, andpressing the pre-molded article in a second die at a molding temperaturehigher than the pre-molding temperature to obtain the separator havingthe final shape.

[0038] Specifically, in the first molding step, it is preferred to set asurface pressure value within a range of 5 to 25 MPa. The first moldingstep can be usually performed at room temperature. Even when the firstmolding step is performed at a heating temperature, it is required thatthe heating temperature is 100° C. or less, Then, the pre-molded articleis set in a heated die, and then pressed to obtain the molded articlehaving the final shape. In the second molding step, it is preferred toset the heating temperature within a range of 130 to 250° C. At thisheating temperature, the pre-molded article can be completely cured. Inaddition, it is preferred that the surface pressure value used in thesecond molding step is determined within a range of 10 to 200 MPa, andparticularly 25 to 200 MPa. Even when using the carbon-phenol resinmolding compound having poor flowability, which is composed of a largeamount of the carbon powder and a small amount of the phenol resin, itis possible to obtain the separator with good quality according to theabove two-stage molding method,

[0039] When reacting the phenol with the aldehyde in the presence of thecatalyst, while mixing those reactants with the carbon powder, a thinoutermost layer of the phenol resin is uniformly formed on the entiresurface of each carbon particle of the carbon powder. Microscopically,there are a lot of fine pores in the phenol resin layer. When the drygranular powder of the molding compound is molded at a heatingtemperature under pressure, the carbon particles is bonded with eachother through the thin phenol resin layer, so that a good electricalconductive state is obtained among the phenol-resin coated carbonparticles.

[0040] In addition, in the present invention, since a uniformly mixedstate of the carbon particles and the phenol resin is readily obtained,it is possible to provide stable quality of the separator for fuelcells. Moreover, the phenol resin on the carbon particles softens at aheating temperature under pressure of the molding step, and then thesoftened phenol resin flows into clearances among adjacent carbonparticles. Therefore, it is possible to substantially avoid theoccurrence of residual pores in the obtained molded article.

[0041] Thus, the carbon-phenol resin molding compound used to form theseparator of the present invention is characterized in that each of thecarbon particles is uniformly coated with the thin phenol-resin layer.Therefore, even when the carbon content in the molding compoundincreases, the carbon particles can be tightly bonded with each other bythe presence of the thin phenol resin layer. As a result, it is possibleto stably provide the separator having excellent mechanical strength andelectrical conductivity and gas impermeability.

[0042] Specifically, even when the carbon content in the moldingcompound is 75 wt % or more, it is possible to obtain the separatorhaving 40 MPa or more of the bending strength, 10×10⁻⁸ cc·cm/cm²s·atm orless of the gas permeability, and 10×10⁻³ Ω·cm or less of resistivity.These properties are particularly adequate for the separator for fuelcells.

EXAMPLES Example 1

[0043] 14 parts by weight of o-cresol, 256 parts by weight of a phenol,380 parts by weight of a 37-wt % formalin, 5.4 parts by weight ofpotassium hydroxide, 1620 parts by weight of a graphite powder, and 1500parts by weight of water were put in a reaction vessel with an agitator.A solubility in water of the o-cresol is 2.0 at normal temperature. Thegraphite powder is of a scale-like powder having an average grain sizeof about 100 μm and containing 95.3 wt % of fixed carbon. A content ofhydrophobic o-cresol in the phenols is 5 wt %. The resultant mixture washeated at 90° C., while agitating. It took 60 minutes to heat themixture to 90° C. The mixture was maintained at 90° C. for 4 hours tofinish the reaction. Subsequently, the reaction product in the reactionvessel was cooled at 20° C., and filtration was performed by use ofNutsche filters to obtain a wet granular material having a water contentof 19 wt %.

[0044] This wet granular material was applied on a polyethylene sheet ina stainless vat to obtain an applied layer having a thickness of about 2cm. The applied layer was dried in a hot-air circulating type dryer atthe temperature of 45° C. for about 48 hours to obtain a dry granularpowder having the water content of 0.7 wt % of the carbon-phenol resinmolding compound. The graphite content in the molding compound is 85.8wt %. The content of the phenol resin in the molding compound is 14.2 wt%. The nitrogen content in the molding compound is 0.02 wt %. Thenitrogen content was measured by Kjeldahl method.

[0045] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 1.

Example 2

[0046] 175 parts by weight of o-cresol, 175 parts by weight of a phenol,480 parts by weight of a 37-wt % formalin, 7 parts by weight ofpotassium hydroxide, 1560 parts by weight of a graphite powder and 1500parts by weight of water were put in a reaction vessel. A solubility inwater of the o-cresol is 2.0 at normal temperature. The graphite powderis of a scale-like powder having an average grain size of about 6 μm andcontaining 95.3 wt % of fixed carbon. A content of hydrophobic o-cresolin the phenols is 50 wt %. Then, according to a similar procedure toExample 1, a wet granular material having a water content of 21 wt % wasobtained.

[0047] Next, as in the case of Example 1, the wet granular material wasdried to obtain a dry granular powder having the water content of 0.7 wt% of the carbon-phenol resin molding compound. The graphite content inthe molding compound is 81.7 wt %. The content of the phenol resin inthe molding compound is 18.3 wt %. The nitrogen content in the moldingcompound is 0.01 wt %. The nitrogen content was measured by Kjeldahlmethod.

[0048] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 2.

Example 3

[0049] 385 parts by weight of a mixed cresol of o-cresol, m-cresol andp-cresol, 490 parts by weight of a 37-wt % formalin, 7.7 parts by weightof potassium hydroxide, 1540 parts by weight of a graphite powder and1500 parts by weight of water were put in a reaction vessel. Asolubility in water of the mixed cresol is 2.0 at normal temperature.The graphite powder is of a scale-like powder having an average grainsize of about 6 μm and containing 95.3 wt % of fixed carbon. The mixedcresol has hydrophobicity. Then, according to a similar procedure to theExample 1, a wet granular material having a water content of 21 wt % wasobtained.

[0050] In addition, as in the case of Example, 1, the wet granularmaterial was dried to obtain a dry granular powder having the watercontent of 0.7 wt % of the carbon-phenol resin molding compound. Thegraphite content in the molding compound is 79.9 wt %. The content ofthe phenol resin in the molding compound is 20.1 wt %. The nitrogencontent in the molding compound is 0.02 wt %. The nitrogen content wasmeasured by Kjeldahl method.

[0051] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 3.

Example 4

[0052] 385 parts by weight of a mixed cresol of o-cresol, m-cresol andp-cresol, 490 parts by weight of a 37-wt % formalin, 7.7 parts by weightof sodium hydroxide, 1540 parts by weight of a graphite powder and 1500parts by weight of water were put in a reaction vessel. A solubility inwater of the mixed cresol is 2.0 at normal temperature. The graphitepowder is of a scale-like powder having an average grain size of about 6μm and containing 95.3 wt % of fixed carbon. The mixed cresol hashydrophobicity. Then, according to a similar procedure to the Example 1,a wet granular material having a water content of 21 wt % was obtained.

[0053] In addition, as in the case of Example, 1, the wet granularmaterial was dried to obtain a dry granular powder having the watercontent of 0.7 wt % of the carbon-phenol resin molding compound. Thegraphite content in the molding compound is 79.7 wt %. The content ofthe phenol resin in the molding compound is 20.3 wt %. The nitrogencontent in the molding compound is 0.02 wt %. The nitrogen content wasmeasured by Kjeldahl method.

[0054] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 4.

Example 5

[0055] 439 parts by weight of a mixed cresol of o-cresol, m-cresol andp-cresol, 560 parts by weight of a 37-wt % formalin, 9.0 parts by weightof trimethylamine, 1370 parts by weight of a graphite powder and 1300parts by weight of water were put in a reaction vessel. A solubility inwater of the mixed cresol is 2.0 at normal temperature. The graphitepowder is of a scale-like powder having an average grain size of about 6μm and containing 95.3 wt % of fixed carbon. The mixed cresol hashydrophobicity. Then, according to a similar procedure to the Example 1,a wet granular material having a water content of 21 wt % was obtained.

[0056] In addition, as in the case of Example, 1, the wet granularmaterial was dried to obtain a dry granular powder having the watercontent of 0.7 wt % of the carbon-phenol resin molding compound. Thegraphite content in the molding compound is 75.7 wt %. The content ofthe phenol resin in the molding compound is 24.3 wt %. The nitrogencontent in the molding compound is 0.02 wt %. The nitrogen content wasmeasured by Kjeldahl method.

[0057] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 5.

Example 6

[0058] The dry granular powder obtained in Example 1 was molded under asurface pressure of about 10 MPa at room temperature to obtain apre-molded article having a shape near the final shape of a separator.Then, the pre-molded article was charged into a required die heated at160° C., and then molded under a surface pressure of about 50 MPa for 3min to obtain the separator for fuel cells of Example 6.

Example 7

[0059] 505 parts by weight of a mixed cresol of o-cresol, m-cresol andp-cresol, 644 parts by weight of a 37-wt % formalin, 10.1 parts byweight of potassium hydroxide, 1300 parts by weight of a graphite powderand 1300 parts by weight of water were put in a reaction vessel. Asolubility in water of the mixed cresol is 2.0 at normal temperature.The graphite powder is of a scale-like powder having an average grainsize of about 6 μm and containing 95.3 wt % of fixed carbon. The mixedcresol has hydrophobicity. Then, according to a similar procedure to theExample 1, a wet granular material having a water content of 21 wt % wasobtained.

[0060] In addition, as in the case of Example, 1, the wet granularmaterial was dried to obtain a dry granular powder having the watercontent of 0.7 wt % of the carbon-phenol resin molding compound. Thegraphite content in the molding compound is 72 wt %. The content of thephenol resin in the molding compound is 28 wt %. The nitrogen content inthe molding compound is 0.02 wt %. The nitrogen content was measured byKjeldahl method.

[0061] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 7.

Example 8

[0062] 347 parts by weight of a phenol, 448 parts by weight of a 37-wt %formalin, 36 parts by weight of hexamethylene tetramine, 1550 parts byweight of a graphite powder, and 1500 parts by weight of water were putin a reaction vessel with an agitator. The graphite powder is of ascale-like powder having an average grain size of about 6 μm andcontaining 95.3 wt % of fixed carbon. Then, according to a similarprocedure to the Example 1, a wet granular material having a watercontent of 21 wt % was obtained.

[0063] In addition, as in the case of Example, 1, the wet granularmaterial was dried to obtain a dry granular powder having the watercontent of 0.7 wt % of the carbon-phenol resin molding compound. Thegraphite content in the molding compound is 81.7 wt %, The content ofthe phenol resin in the molding compound is 18.3 wt %. The nitrogencontent in the molding compound is 0.9 wt %. The nitrogen content wasmeasured by Kjeldahl method.

[0064] The obtained dry granular powder was charged into a required dieheated at 160° C., and then molded under a surface pressure of about 25MPa for 3 min to obtain a separator for fuel cells of Example 8.

Comparative Example 1

[0065] 240 parts by weight of a powder-like alkali-resol type phenolresin were ball-milled, and then a required amount of methanol was addedto the ball-milled phenol resin to obtain a slurry. Next, 760 parts byweight of a graphite powder was added to the slurry and then a resultantmixture was agitated by use of a kneader. The graphite powder is of ascale-like powder having an average grain size of 100 μm and containing95.3 wt % of fixed carbon. After the resultant mixture was dried at 60°C., a small amount of magnesium stearate was added and mixed by use of amixer to obtain a carbon-phenol resin molding compound. The graphitecontent in the molding compound is 76 wt %, The content of the phenolresin in the molding compound is 24 wt %. The nitrogen content in themolding compound is 0.02 wt %.

[0066] The molding compound was charged into a required die heated at160° C., and then molded under a surface pressure of about 25 MPa for 3min to obtain a separator for fuel cells of Comparative Example 1.

[0067] (Evaluation)

[0068] Compositions of the molding compounds of Examples 1 to 8 andComparative Example 1 are shown in Table 1. With respect to the moldingcompound of each of Examples 1 to 8 and Comparative Example 1, bendingstrength, resistivity, gas impermeability and EMF (electromotive force)characteristics were measured. That is, the bending strength wasmeasured by use of a specimen (10 mm×4 mm×80 mm) according to the methoddescribed in JIS (Japanese Industrial Standards) K 7171. The resistivitywas measured by use of a test piece having a thickness of 2 mm accordingto the method described in JIS K 7194. The gas impermeability wasdetermined by providing a space filled with nitrogen gas under anatmospheric pressure at a side of the test piece, and measuring anamount of nitrogen gas transmitted through the test piece at theopposite side of the test piece. In addition, the EMF characteristicswere measured by preparing a fuel cell (stack number: 2), and measuringan electromotive force between a pair of cells, while running a coolingwater (antifreeze liquid: ion exchange water 50 wt %). In Table. 2, thesymbol “◯” designates that a reduction in EMF did not happened, and thesymbol “X” designates that the reduction in EMF happened. Results areshown in Table 2.

[0069] As shown in Table 1, the separator of each of Examples 1 to 8 hassmaller resistivity and higher electrical conductivity in comparisonwith the separator of Comparative Example 1. In addition, the separatorsof those Examples exhibit excellent bending strength and gasimpermeability. Since the carbon content in the molding compound ofExample 7 is relatively small, the electrical conductivity is slightlylower in comparison with the other Examples. In addition, since thenitrogen content in the molding compound of Example 8 is relativelylarge, a reduction in EMF happened. TABLE 1 Hydrophobic Phenols phenolAldehyde Catalyst Example 1 Phenol o-cresol 5 Formalin KOH Example 2Phenol o-cresol 50 Formalin KOH Example 3 — mixed cresol 100 FormalinKOH Example 4 — mixed cresol 100 Formalin NaOH Example 5 — mixed cresol100 Formalin Triethylamine Example 6 Phenol o-cresol 5 Formalin KOHExample 7 — mixed cresol 100 Formalin KOH Example 8 Phenol — 0 FormalinHexamethylene- tetramine Comparative Phenol — 0 Formalin KOH Example 1

[0070] TABLE 2 Graphite Bending Nitrogen Resin Powder StrengthResistivity Gas Transmittance constituent Reduction (wt %) (wt %) (MPa)(Ω · cm) (cc · cm/cm²s · atm) (wt %) in EMF Example 1 14.2 85.8 44 4.8 ×10⁻³ <6 × 10⁻⁸ 0.02 ∘ Example 2 18.3 81.7 47 6.3 × 10⁻³ <5 × 10⁻⁸ 0.01 ∘Example 3 20.1 79.9 51 8.2 × 10⁻³ <5 × 10⁻⁸ 0.02 ∘ Example 4 20.3 79.753 8.3 × 10⁻³ <5 × 10⁻⁸ 0.02 ∘ Example 5 24.3 75.7 58 9.8 × 10⁻³ <5 ×10⁻⁸ 0.02 ∘ Example 6 14.2 85.8 58 4.9 × 10⁻³ <5 × 10⁻⁸ 0.02 ∘ Example 728 72 60 1.2 × 10⁻² <5 × 10⁻⁸ 0.02 ∘ Example 8 18.3 81.7 50 5.9 × 10⁻³<5 × 10⁻⁸ 0.9  x Comparative 24 76 42 1.6 × 10⁻²  5 × 10⁻⁸ 0.02 ∘Example 1

What is claimed is:
 1. A separator for fuel cells, which is a moldedarticle of a carbon-phenol resin molding compound obtained by reacting aphenol with an aldehyde in the presence of a catalyst, while mixing witha carbon powder.
 2. The separator as set forth in claim 1, wherein themolded article is of a thin-plate shape having a flow channel.
 3. Theseparator as set forth in claim 1, wherein a carbon content in themolding compound is within a range of 75 wt % to 97 wt %.
 4. Theseparator as set forth in claim 1, wherein said catalyst is at least oneselected from tertiary amines, carbonates, hydroxides and oxides ofalkali metals or alkali earth metals.
 5. The separator as set forth inclaim 1, wherein a content of nitrogen constituent in the moldingcompound is 0.3 wt % or less.
 6. The separator as set forth in claim 1,wherein said carbon powder contains 90 wt % or more of fixed carbon.